The present invention relates to an apparatus for determining tissue elasticity in various parts of the body and using such information as a diagnostic tool in the detection of abnormalities of tissue, such as those caused by cancer or other lesions. The "hardness" of tumors can be quantified in terms of the surrounding tissue elastic properties.
Diagnosing early formation of tumors or lesions, particularly those caused by cancer, has been a problem that has been attempted to be solved using various techniques, such as ultrasonic imaging, nuclear magnetic resonance imaging, x-rays, and the like. Each of these techniques have limitations, including the application of radiation to the body, which may be harmful to the body being tested.
One approach attempts to determine the relative stiffness or elasticity of tissue by applying ultrasound imaging techniques while vibrating the tissue at low frequencies. See. e.g., R. M. Lerner et al., Sono-Elasticity: Medical Elasticity Images Derived From Ultrasound Signals in Mechanically Vibrated Targets, Acoustical Imaging, Vol. 16, 317 (1988), Robert M. Lerner et al., "Sonoelasticity" Images Derived from Ultrasound Signals in Mechanically Vibrated Tissues, Ultrasound in Med. & Biol. Vol. 16, No. 3, 231 (1990) and K. J. Parker et al., Tissue Response to Mechanical Vibrations for "Sonoelasticity Imaging", Ultrasound in Med. & Biol. Vol 16, No. 3, 241 (1990).
A variety of other methods have been proposed for measuring the mechanical characteristics, e.g., elasticity, inside soft tissues. One method includes using an ultrasonic wave as a probing wave to observe the mechanical responses of tissues due to cardiac pulsation. The mechanical responses are observed using the ultrasonic wave and then information regarding the mechanical characteristics are estimated on patterns of small movements in the tissue in response to cardiac pulsation. See R. J. Dickinson and C. R. Hill, Measurement of Soft Tissue Motion Using Correlation Between A-Scans, Ultrasound in Med. and Biol. Vol. 8, 263 (1982) and M. Tristam et al., Ultrasonic Study of In Vivo Kinetic Characteristics of Human Tissues, Ultrasound in Med. and Biol. Vol. 12, 927 (1986). The technique uses Fourier analysis to objectively differentiate different tissue types in pathologies based on numerical features of the time-course of a correlation coefficient between pairs of A-Scans recorded with a particular time separation. Tissue oscillations resulting from ventricular contraction and pressure pulses in the descending aorta are measured to derive patterns of movement. Fourier series transformation is used to analyze the data to quantitate the kinetic behavior of the tissue in vivo. M. Tristam et al., Application of Fourier Analysis to Clinical Study of Patterns of Tissue Movement, Ultrasound in Med. and Biol. Vol. 14, 695 (1988).
Another method for estimating the mechanical properties of desired points inside the tissue has been proposed in which low-frequency vibration is applied to the surface and the wave velocity inside the tissue is measured by using the simultaneously transmitted ultrasonic waves. The difference of velocity near the measuring point allows one to derive the elastic property of the tissue. See T. A. Krouskop et al., A Pulsed Doppler Ultrasonic System for Making Non-Invasive Measurement of Mechanical Properties of Soft Tissue, 24 J. Rehab. Res. Dev. Vol. 24, 1 (1987).
Another method of evaluating the elasticity of tissue includes applying low-frequency vibration (e.g., several hundred Hz) to the surface while measuring both the amplitude and phase of internal vibration based on Doppler frequency modulation of simultaneously transmitted probing ultrasonic waves. The amplitude and phase maps are used to observe information that relates to the viscoelastic properties of the tissues. Y. Yamakoshi et al., Ultrasonic Imaging of Internal Vibration of Soft Tissue Under Forced Vibration, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 7, No. 2, Page 45 (1990).
In another method, the wave forms of liver dynamics caused by aortic pulsation and vessel diameter variations are observed by a signal processing technique for analyzing radio frequency M-mode signal patterns of movement (in the liver) in response to the arterial pulsation. The wave forms are used to determine tissue characteristics (displacement, velocity, and strain) as a function of time in small deformations in tissue due to the arterial pulsation. Wilson and Robinson, Ultrasonic Measurement of Small Displacements and Deformations of Tissue, Ultrasonic Imaging Vol. 4 (1982) 71-82.
Another method recently proposed for measuring and imaging tissue elasticity is described in Ophir et al., U.S. Pat. No. 5,107,837. This method includes emitting ultrasonic waves along a path into the tissue and detecting an echo sequence resulting from the ultrasonic wave pulse. The tissue is then compressed (or alternatively uncompressed from a compressed state) along the path and during such compressing, a second pulse of ultrasonic waves are sent along the path into the tissue. The second echo sequence resulting from the second ultrasonic wave pulse is detected and then the differential displacement of selected echo segments of the first and second echo sequences are measured. A selected echo segment of the echo sequence, i.e., reflected RF signal, corresponds to a particular echo source within the tissue along the beam axis of the transducer. Time shifts in the echo segment are examined to measure compressibilities of the tissue regions. This technique is further described in Ophir et al., Elastography: A Quantitative Method for Imaging the Elasticity of Biological Tissues, Ultrasonic Imaging 13, 111 (1991). See also J. Ophir et al., A Transaxial Compression Technique (TACT) For Localized Pulse-Echo Estimation of Sound Speed in Biological Tissues, Ultrasonic Imaging 12, 35 (1990).
It is desirable to have the capability to investigate tissue elasticity changes, which may indicate precursors of tumors or actual tumors without subjecting the patient to radiation. There is also a need for equipment that is easy to use and which requires relatively low capital investment. It also would be desirable to use currently available non-invasive imaging modalities, such as ultrasound, magnetic resonance imaging (MRI), computer aided tomograph (CAT) scanning, and the like.